1. Field of the Invention
This invention relates to the field of intrusion alarms of the type electrically sensitive to the presence of a would-be intruder and arranged to sound an alarm whenever an electrical condition exists which corresponds to the presence of the intruder.
2. Description of the Prior Art
A variety of different intrusion alarms are known. Some prior art alarms are designed to be used in association with a doorknob apparatus to detect the presence of a would-be intruder, as exemplified by U.S. Pat. No. 3,508,239 to Fontaine, U.S. Pat. No. 3,774,191 to Enemark, and U.S. Pat. No. 3,824,576 to Pioch. A variety of sensing techniques also are known, as shown by the following U.S. patents:
______________________________________ Mallory 1,870,181 Browning 2,421,771 Ripepi 3,022,499 Premack 3,222,664 Rode 3,128,416 Lister 3,510,677 Ralston 3,778,807 ______________________________________
Despite the abundance of known intrusion alarm devices, existing intrusion alarms are unsatisfactory in many ways. For example, existing intrusion alarms require time consuming installation, and thus are not suitable for use by travellers to guard the security of hotel doors and the like. Existing intrusion alarms, moreover, are passive except when an intruder appears, and are never to be fully relied upon since a failure would go unnoticed. In addition, existing intrusion alarms are somewhat awkward to use since, when set in an active state, they require the user to bypass the monitored apparatus completely to avoid sounding the alarm.